The present invention relates to an image synthesizing apparatus for synthesizing two pieces of image data and outputting synthesized image data, and particularly to an image synthesizing apparatus for effecting image synthesis by using an α blending method.
Recently, functions of a television receiver have been diversified. Since conventional mechanical switches such for example as buttons provided on the body of the television receiver and a remote control are not sufficient for setting the functions of the television receiver, the functions are more often set by menu operation such as displaying a menu image on a monitor screen, selecting a setting item, and supplying an input on a displayed setting screen. In some of such cases, an image for menu operation is superimposed on a television image currently selected for viewing by a television tuner on the monitor screen.
FIG. 7 shows an example of a display screen of a television receiver for such menu operation.
FIG. 7 shows a menu screen 71 to be superimposed, a television screen 72 selected by a television tuner or the like, and a synthesized screen 73 in which the menu screen 71 is superimposed on the television screen 72. Selection buttons 71a, 71b, and 71c shown on the menu screen 71 for example represent a “sound selection” button for switching between a main-channel sound and a sub-channel sound, a “channel” button for setting the display number of a channel, and a “return” button for stopping the display of the menu screen, respectively.
The television screen 72 being viewed can be seen through the menu selection buttons 71a, 71b, and 71c on the synthesized screen 73 in which the menu screen 71 is superimposed on the television screen 72. An image synthesizing method that makes one image seen through the other image in such a manner is referred to as α blending. This processing method is often used for menu operation on a television screen.
According to the α blending method, when an image to be superimposed is set to be A, a background image is set to be B, and a blending coefficient α is set to be α(0≦α≦1), an operation is performed on each element forming respective one of the images A and B so that an output image S satisfies the following equation, and then the result is outputted.S=A*α+B*(1−α)An arbitrary value in a range of 0≦α≦1 is selected for the blending coefficient α to thereby define transparency; when α is zero, the image A to be superimposed on the image B is completely transparent, while when α is unity, the image A to be superimposed on the image B is completely opaque.
When the menu screen is superimposed on the television screen by using the α blending method as described above, the blending coefficient α is generally determined for each menu screen using a table referred to as a color look-up table (hereinafter referred to as a CLUT).
FIG. 8 shows an example of a CLUT. In FIG. 8, Y, Cb, and Cr are picture element components that form a picture element, and Y represents a luminance component while Cb and Cr represent color difference components. Coordinate values represent coordinates (x, y) of a picture element on a monitor screen obtained when an x-component is taken in a horizontal direction and a y-component is taken in a vertical direction. In order to simplify description, it is assumed that only four sets of coordinates are present in FIG. 8. The values of the luminance component Y, the color difference component Cb, the color difference component Cr, and α are converted into 8-bit data for convenience of operation.
In order to change transparency of a synthesized screen for each picture element, the α blending method using a CLUT requires information on an α value for each picture element, as shown in FIG. 8. Therefore, the finer the picture elements on a monitor screen, the larger the amount of information of the CLUT, thereby increasing memory size and the load of the above-mentioned operation processing, and thus decreasing image-generating rate. In order to deal with the problem, circuit scale, power consumption, and cost of the television receiver must be increased.